The present invention relates to thermal bend actuators and more particularly to a bend actuator structure that is suitable for use in the design of low powered generally elongate mechanisms, which using prior art structures may have been prone to buckling during operation.
The invention has been developed primarily for use with transient thermal bend actuators, that is actuators utilising parallel conductor beams of identical materials so as to be sensitive primarily to relative temperature differences and thereby less sensitive to ambient conditions, and will be described hereinafter with reference to this preferred application. However, it will be appreciated by those skilled in the art, that the inventive concept is equally applicable to non-transient thermal bend actuators including, for example, bimetallic actuators including electro mechanical thermal switches.
The present invention has been developed as a means of overcoming problems commonly encountered during the manufacture and design of micro-electro mechanical systems (MEMS), which are produced via a multi-stage process of successively depositing and etching thin film layers of different materials using integrated circuit CMOS technology.
Typically, transient thermal bend actuators used in MEMS are formed by depositing interconnected parallel spaced apart thermally isolated layers or beams of conductive material within surrounding layers of non-conductive structural material such as silicon. The beams are secured at one end at an anchor and connected at the other to the movable part of the mechanism. The actuator is then controlled by the passage of a heating current through one of these layers known as the active beam member.
In order to minimise the power required to operate the mechanisms incorporating these actuators, it is desirable to use relatively long actuating arms so as to achieve a balance between magnifying the maximum displacement at the free distal end of the lever mechanism, achieving the requisite applied force, and controlling the voltage and amount of current required to effectively operate the actuator.
It has been found when modelling such devices, that, not unexpectedly, the longer the unsupported span of the parallel beam members of the actuator, the more likely that some degree of buckling is going to occur, the mode being determinable by a number of factors. Whilst it is possible to predict the buckling mode and come up with satisfactorily operational designs that allow for such buckling, this is likely to result in a less efficient and/or less predictable mode of operation of the device.
Accordingly, it has been preferable prior art practice to try and stiffen the effective beam portion of the actuator to minimise the buckling. This can be done in a number of ways including forming stiffening struts and other reinforcing structures that extend transversely between and often beyond the two actuator beams. However, this can have two disadvantages. Firstly, the additional structural material and general stiffening of the lever may result in higher operational power requirements. Further, the additional mass of the structure contacting the active beam member may act as a heat sink, increasing the thermal losses during heating, thereby again increasing the power required to operate the device.
It is an object of the present invention to provide a thermal bend actuator structure which overcomes or at least ameliorates one or more of the above discussed disadvantages of the prior art, or which at least offers a useful alternative.
According to a first aspect of the invention there is a provided a thermal bend actuator including:
a first anchor portion for securing to a fixed substrate;
a first thermally conductive active beam member secured at a proximal end to said first anchor portion and extending to a movable distal end;
a second beam member similarly anchored at a proximal end to said first anchor portion so as to extend parallel to said first active beam member, each of said first and second beam members being directly or indirectly interconnected at their respective distal ends remote said first anchor portion;
wherein said first thermally conductive active beam member is configured to define a labyrinthine conductive pathway having a combined effective length in a direction extending between said fixed proximal end and said movable distal end that exceeds the effective direct linear path therebetween.
The term xe2x80x9ceffective lengthxe2x80x9d is used to refer to the heating portion of the conductive pathway over which bend inducing thermal expansion occurs.
In a first preferred form, the labyrinthine pathway is configured to define a plurality of parallel strips oriented to extend in a direction that is parallel to the direction between said respective proximal and distal ends, said strips being serially interconnected to define a single pathway. As this pathway in the preferred embodiment is formed on a thin film layer, this results in a square wave type pattern extending across the active beam member.
Preferably, the labyrinthine pathway starts and ends at the proximal end of the active beam adjacent the anchor portion, so that power need only be supplied to the active beam member from the fixed portion of the mechanism. In the first preferred form, this results in a structure having an even number of parallel serially interconnected strips.
Desirably, the first active beam member and/or the second beam member each has a first anchor element formed as one or more tabs at its proximal end, and a second anchor element, similarly preferably formed as one or more tabs for securing to the movable lever of the device to which the actuator is to be connected.
Preferably, the thermal bend actuator is a transient thermal bend actuator in which the second beam member is formed from a identical material to that of the first member and which has substantially identical physical configurations along the portions that extend parallel to the operational portion of the first active beam member, the ends of the beams remote the anchor portion being held in a spaced apart relationship by means of an intermediate non conducting material extending therebetween.